Various hydrogels have been investigated as a potential approach for the delivery of various active ingredients, especially pharmaceutical and other bioactive materials. For example, microgels and microparticles made of acrylamides and methacrylamides formed using an aryl cross-linker capable of delivering bioactive materials to cells have been described in Frechet et al., U.S. Pat. No. 7,056,901, which is incorporated by reference. In many instances, it is preferable that the hydrogels release their contents in response to an environmental stimuli, allowing for the targeting of protein therapeutics to diseased tissues and cells. A particularly important environmental stimulus is mildly acidic pH. For example, tumors exist at acidic pHs between 6.4 to 6.8, and the phagolysosomes of phagocytic cells are at pHs between about 4.5 to 5.0. The acidic nature of these compartments has stimulated a need for the development of hydrogels that can selectively release their contents under mildly acidic conditions.
In addition, hollow particles, also referred to as capsules or vesicles, are generating interest in industry, as well as scientific research. For example, Feldheim et al., U.S. Pat. No. 6,602,932, which is incorporated by reference, describes polypyrrole nanoparticle composites and nanocapsules which encapsulate a “guest molecule.”
While there has been significant progress, there remains a need to develop additional novel nanoparticles, nanogels, composite nanoparticles, and nanocapsules useful in the delivery of various active ingredients.